1. Field of the Invention
The invention relates to a method and a system for monitoring of the temperature of the surface or the outer skin of an aircraft due to heat radiation.
2. Discussion of Background Information
Such methods are already known in the prior art and serve to determine the surface temperature of an aircraft located in particular on the ground, in order to ensure that a maximum operating temperature specified when designing the aircraft is not exceeded.
In a commonly used method, the maximum occurring operating temperature is determined when designing the aircraft by way of calculations, measurements, and statistical evaluation. It is thereby ensured with a high probability that the aircraft does not actually exceed this operating temperature in reality. However, this method includes the usual statistical uncertainties, due to which either the temperature to be assumed is too high (with the ensuing conservative design) or on the contrary, the probability of real temperatures occurring, which exceed the specified operating temperature, is too high. The statistical basis for the determination of the temperatures depends, inter alia, on wind and solar intensity data. These are dependent on the geographical location of the possible airports, which can control the aircraft. If, after designing the aircraft and determining the maximum operating temperature, a new airport becomes available which would give rise to higher operating temperatures, there would be the risk that this should have been taken into account when designing the aircraft. Such imponderables can only be reliably avoided by very high assumed maximum operating temperatures, which, however, leads to forfeiture of an optimum design for the aircraft.
It is further known, in one modification of the method, to take into account the influence of the selected aircraft paints. By restricting the shades and/or by using special thermal-radiation-reflecting paints (also called “solar heat reflectant paints”), the maximum temperature to be taken into account can be reduced to an acceptable level. However, the restriction of the paint both in shade and in paint material constitutes an undesirable hindrance for the aircraft operator. Furthermore, when applying such a method, it is difficult to check whether the relevant aircraft stay painted with the correct paints throughout their entire operating life. The availability of the paints may be cited as a further disadvantage since the allowed paints may well no longer be available after a few years. As a last aspect, the temporal stability of the paints is also questionable. It is uncertain whether after some time, a solar heat reflectant paint still reflects solar radiation just as effectively as immediately after it was originally applied to the aircraft. For these reasons, it ultimately follows that this method is certainly applicable but does not appear to be particularly attractive and is beset with a number of uncertainties.
Another method pursues a different approach and uses fixedly installed temperature sensors to measure temperatures which actually occur at critical positions on the aircraft surface and prevents operation of the aircraft by corresponding displays in the cockpit which indicate that a specified maximum operating temperature of the aircraft is exceeded. The application of fixedly installed sensors has the disadvantage that changes in the color design could result in a change in the temperature distribution, so that possibly the fixedly installed sensors no longer detect the areas having the maximum temperature, so that their position needed to be changed. The latter would lead to appreciable expenditure since the internal lining and insulation of the aircraft would need to be partially removed to wire the sensors accordingly. Furthermore, in view of the adverse influence on the outside of the aircraft and the susceptibility of the sensors to breakdown, false alarms could result, which in turn cause loss of income for the operators. Alternatively, the use of a plurality of redundant sensors at one location or the use of very reliable sensors results in high costs.
Document WO 93/01 977 A1 describes a system for the visual determination of damages or incorrect operation which is externally observable on an airplane especially during flight. In this regard an infrared sensor is used for determination of radiation of a region of the outer airframe of the airplane. The system generates a pattern of pixels of the region and assigns a predetermined relevant parameter to each pixel of the same and compares this parameter with a value measured by the sensor. The system generates a warning signal for a display when the value which is measured for a pixel deviates from a value predetermined for this pixel.
Document U.S. Pat. No. 4,816,828 describes a system for visual determination of damages or incorrect operation which are externally observable on an airplane especially during flight.
Further, document DE 10 2006 031 09 A1 discloses a method for monitoring of the state of structural components in which an optical sensor in combination with a computer determines from subsequent images form the structural components to be monitored deviations between images and therefrom structural changes.